Fuel in gaseous form is widely used for power plants. FIG. 1 illustrates a simplified view of a gas turbine power plant 100. The plant 100 includes a coalescing filter 110, a performance heater 120, a startup heater 130, a gas scrubber 140, and various valves including ASV (auxiliary stop valve) 150, SRV (speed ratio valve) 160, and GCV (gas control valve) 170, and a gas turbine 180.
For operational and safety reasons, fuel gas delivered to the gas turbine should be free of liquids such as moisture from water and hydrocarbon liquids. Moisture is undesirable since it can combine with hydrocarbons such as methane to form solid hydrates which can damage the turbine. Hydrocarbon liquids are of particular concern since they can lead to undesirable situations such as uncontrolled heat addition, gas turbine over speed, auto ignition, flash back and re-ignitions among others.
The fuel gas supplied to the power plant is normally pressurized and undergoes pressure reduction and expansion as the fuel gas flows through the valves 150, 160, 170 before being provided to the turbine 180. Gas cooling, also referred to as the Joule-Thomson effect, occurs as a result of this pressure reduction and gas expansion. If substantial cooling occurs, water and liquid hydrocarbon condensation can form presenting risks as described above.
To avoid condensation from forming, the fuel gas is superheated upstream of the valves 150, 160, 170. That is, the fuel gas is heated sufficiently above the moisture and hydrocarbon dew points. By superheating the fuel gas upstream, condensation is prevented downstream when the fuel gas is depressurized and expanded as the gas flows through the valves 150, 160, 170. This ensures that the only the fuel in gaseous form is provided to the inlet of the gas turbine 180.
As illustrated in FIG. 1, two types of heaters—the performance heater 120 and the startup heater 130—are typically used to superheat the fuel gas. During normal operation, the performance heater 120 uses hot water as a heat source to superheat the fuel gas. The hot water is provided from a heat recovery steam generator (HSRG) (not shown in FIG. 1) which in turn uses the hot exhaust from the gas turbine 180 to heat the water.
When the gas turbine 180 is in startup operation, hot water from the HSRG is not yet available. Until the gas turbine 180 reaches normal operation, the startup heater 130 provides the necessary superheating. Electric heaters are typically used due to their relative simplicity and convenience. An external view of a conventional electric startup heater is illustrated in FIG. 2. The conventional startup heater can be very sizable even for a plant that generates a modest amount of power. As an example, the skid for the conventional startup heater can be as big as 15 ft (length)×8 ft (width)×12 ft (height) for a gas turbine power plant with 0.28 megawatt generating capacity.
In the conventional electric startup heater, the active heating elements are mounted inside the piping (not shown) where the fuel gas flows. This allows for high heat transfer to take place so that the fuel gas temperature can be elevated quickly.
It will be appreciated that the conventional electric startup heater is prone to safety issues due to the heating elements being in direct contact with the fuel gas. Since the heating elements are in the path of the fuel gas flow, the heating elements themselves add to an undesirable pressure drop. Service and upgrade is made difficult due to the internal placement of the heating elements, which require valves, gaskets, and other fixtures so that the pipes containing the heating elements can be isolated for the service and upgrade. Operationally, it may be required that the fuel gas be flowing while the heating elements are on to reduce the possibility of heating element failure. Also, the conventional electric startup heater is very expensive—one system can be in excess of $265,000. There remains a need therefore, for system(s) and/or method(s) for superheating the fuel gas during system startup that reduces, if not eliminates, the disadvantages associated with the conventional electric startup heater.